The objective of this research is to obtain accurate structures of lipid bilayers, especially regarding the area per molecule and the thickness of the hydrophobic layer. Such structures will provide a data base for evaluating how different lipids affect biomembrane function, including passive transport and protein-lipid interaction and, also, for evaluating the various interaction energies that determine the structure of membranes. The research utilizes a combination of diffraction and volumetric data. The volumetric data is being obtained using the neutral flotation technique. The diffraction data is being obtained using a rotating anode source with a variety of detectors, including a high precision scintillator-photomultiplier detector, a linear position sensitive detector and film. The use of new electron density models allows the incorporation of both low angle and wide angle x-ray and neutron diffraction data and the volumetric data into the same analysis. New quantitative mathematical relations connect the information perpendicular to the bilayer to information lateral to the bilayer. X-ray diffraction measurements will be made on fully hydrated unoriented multilamellar liposomes, oriented multilamellar liposomes, and large and small unilamellar vesicles to test hypotheses regarding the effect of interlamellar interactions on the structure of lipid bilayers. Determination of the structure of the biologically relevant chain disordered phases will be aided by bootstrapping from the determination of the structure of the chain ordered phases that occur at lower temperature.